Zeolites have been available for many years. Zeolites are alumina silicate complexes formed of layers of ring structures. The resulting structure has a controlled pore size which may include or exclude molecules of different sizes. Different zeolites having different ratios of aluminum to silica have a different unit structure on a molecular level and tend to have a different pore size.
U.S. Pat. No. 6,858,556 published Feb. 22, 2005 in the name of Kuvettu, et al., assigned to the Indian Oil Corporation Limited, discloses a process for cracking heavier feedstocks in the presence of a stabilized dual zeolite catalyst having a particle size in the range of 30-100 microns to produce a gasoline fraction and a liquefied petroleum gas (LPG) fraction typically lower alkanes (e.g. ethane, propane and butane). The patent does not suggest using a high pore volume component in the zeolite nor does the patent teach producing olefins.
EP application 0 925 831 published Jun. 30, 1999 to Guan et al., assigned to China Petrochemical Corporation and Research Institute of Petroleum Processing, Sinopec teaches a method for cracking heavy oil. The oil is cracked in a fluid bed cracker in the presence of a catalyst comprising one or more zeolites and a pillared clay. The present invention has eliminated the essential feature of a pillared clay from the '831 art. The zeolites are conventional zeolites and have not been treated with alkali. The '831 application does not disclose or suggest the subject matter of the present invention.
U.S. Pat. No. 3,894,934 issued Jul. 15, 1975 to Owen et al., assigned to Mobil oil Corporation teaches cracking a hydrocarbon feed in the presence of a small pore zeolite and a large pore zeolite in a weight ratio of 1:10 to 3:1. The small pore zeolite has a pore size not exceeding 9 angstroms (0.9 nanometers) and the large pore size zeolite has a pore size greater than about 9 angstroms (0.9 nanometers) (Col. 3 lines 30-35). The feed has an initial boiling temperature from 400° F. to 1100° F. (204° C. to 594° C.) to produce a gasoline cut and a lower paraffin or olefin stream which can be used to enhance the octane number of the resulting gasoline stream. The subject matter of the '934 patent teaches away from the subject matter of the present invention.
Ogura et al. (Masaru Ogura, Shin-ya Shinomiya, Junko Tateno, Yasuto Nara, Mikihiro Nomura, Eiichi Kikuchi, Masahiko Matsukata, Applied Catalysis A: General, 2001, 219, 33-43) found that a morphological change of ZSM-5 particles by the alkali-treatment could be observed and many cracks and faults were formed on the outer surface of zeolite grains or particles. Mesopores having a uniform size were formed in the zeolite particles, although the microporous structure remained under the conditions used in their work. In addition, they also found that the catalytic activity for cumene cracking was enhanced by the treatment. Their result indicated that alkali-treatment led to an increase in the number of adsorption sites and also in the diffusivity of benzene through zeolite micropores. The enhancement in catalytic performance can be explained due to the fact that the adsorptive-diffusive property of ZSM-5 is improved by alkali-treatment.
Suzuki et al. (Tetsuo Suzuki, Toshio Okuhara, Microporous and Mesoporous Materials, 2001,43, 83-89) declared that the NaOH-treatment of MFI zeolite brought about the increases in total surface area and external surface area. The increase in the surface areas was due to the formation of the supermicropores having about 1.8 nm in diameter, while the ultramicropores remained almost unchanged in the size and the volume. The supermicropores would be formed by the dissolution of tallites of MFI. The rate-determining step of dissolution of MFI zeolite would be the diffusion process of NaOH aqueous solution into the newly formed supermicropores.
Groen et al. (J. C. Groen, L. A. A. Peffer, J. A. Moulijn, J.Pérez-Ramirez, Colloids and Surfaces A: Physicochem. Eng. Aspects, 2004, 241, 53-58) studies are focused on the evolution and optimization of the porous structure by varying treatment time and temperature, using N2- and Ar-adsorption. N2-adsorption experiments have shown that optimization of the alkaline treatment of ZSM-5 zeolite leads to a combined porous material with an increased mesoporosity and preserved microporosity. An optimal treatment of commercial ZSM-5 (SiO2/Al2O3=37) in 0.2M NaOH at 338 K for 30 min results in a spectacular increase of mesopore surface area from 40 to 225 m2/g (˜450%) and a relatively small decrease in microporosity (25%). The mesopore formation is a result of preferential dissolution of Si from the zeolite framework. Variation of treatment time and temperature enables a certain tuning of the mesopore-size and volume. XRD (X ray diffraction) analysis evidences the long-range ordering to remain intact, while low-pressure Ar-adsorption confirms the preserved microporosity in the optimal alkaline-treated zeolite. Controlled desilication in ZSM-5 by an optimal alkaline treatment opens new approaches in the development of combined micro- and mesoporosity in catalyst design.
It is known, ZSM-5 having a lower framework (structure) silica to alumina (e.g. SiO2/Al2O3) ratio can be obtained from the alkali treatment of ZSM-5. To synthesize a zeolite having an ultra-low framework Si/Al ratio is not easy. It will require a longer time, higher temperature with lower crystallinity and yields. During the desilication process, the lower SiO2/Al2O3 ratio can be obtained through extracting siliceous species from the framework of ZSM-5. These results have also been reported by Ogura et al. (Masaru Ogura, Shin-ya Shinomiya, Junko Tateno, Yasuto Nara, Mikihiro Nomura, Eiichi Kikuchi, Masahiko Matsukata, Applied Catalysis A: General, 2001, 219, 33-43) and Wang D Zh et al. (Wang D Zh, Shu Xingtian, He Mingyuan, Chinese Journal of Catalysis, 2003, 24(3), 208-212.).
The present invention seeks to provide a catalyst based on mixed and modified zeolites, preferably alkali modified to reduce the Si/Al ratio and increase the pore size/volume (meso-large pore BET surface area (defined as the total BET surface area minus the micro-pore BET surface area) of greater than 50 m2/g) suitable for cracking a heavy oil feed to produce a high amount of lower (C2-4) olefins, and a gasoline and diesel cut which catalyst has a lower propensity for coking.